There are various industrial applications where a workpiece has a throughbore which is formed by two counterbores where one of the bores may be particularly subject to wear. Such excessive wear may be brought about by the material that the workpiece is made of or the action of whatever operative components work in the bore or a combination of these and perhaps other factors. Particularly in regard to workpieces which may be complexly configured such that the original manufacturing may make the workpiece relatively expensive, it is highly desirable if a damaged bore in an otherwise usable workpiece can be repaired or in some manner reconditioned.
One such device of this type exists in the automotive field in the form of the main housing for many power rack and pinion steering assemblies. The power rack and pinion steering housing as manufactured by or for several automotive manufacturers consists of an elongate, irregularly shaped housing which seats a spool valve having a plurality of peripheral grooves which interact with a plurality of ports in the housing. Since the housing is constructed of aluminum for weight reduction purposes the pressurized fluid in the spool valve grooves eventually forms score lines or recesses in the housing. At some point in time the score lines become sufficiently pronounced such that fluid leakage between the various spool valve grooves takes place to an extent that the operation of the power assist to the steering mechanism is adversely affected.
Due to the significant costs involved in replacement of the entire housing, efforts are being made in the vehicle repair industry to recondition or rebuild the housings. The housings are generally an elongate member for receiving a shaft having the grooved spool valve at one end and a fluted gear near the opposite end for engaging a rack. The bore for the shaft is counterbored from both ends with one bore designed to receive the spool valve and supporting bushings, bearings and seals and the other end accommodating the fluted gear portion of the shaft. As a basic manufacturing technique a pair of counterbores are made to receive these two portions of the shaft. According to standard manufacturing techniques the two bores in the counterbored housing are not necessarily exactly concentric. Any effort to rebore or to rebore and fit the bore housing the spool valve with a metallic sleeve necessarily contemplates that the rebore be made with a reamer which is exactly concentrically aligned with the original bore. If this alignment is not achieved, the shaft or rod mounting the spool valve will be misaligned with and therefore bind in the associated bushings and bearings on which it rides. Even minor misalignment produces a defective power steering unit which cannot be used successfully as a replacement part.
As a result various attempts have been made to devise a method and apparatus for reboring such steering housings with concentric alignment achieved to within the requisite tolerances of approximately one half a thousandths of an inch. For the most part these efforts at repairing the housings by reboring and inserting a sleeve have proven to be costly in terms of requiring a substantial amount of time of skilled machinists and proving difficult to maintain within required tolerances, thus producing numerous scrap parts or rebuilt parts with marginal operating parameters.
One approach to endeavoring to rebore such housing involves utilization of the original tooling which was employed to hold the housing to make the original bore comply with use of a larger reamer. It is believed that this has proved to be largely unsuccessful evidently for the reasons that variations in different versions of the same tooling and variations in the outer dimensions of the housing are of a sufficient magnitude such that the requisite precision in achieving concentric alignment with the original bore cannot be repeatedly achieved. Thus, the concept of reprocessing damaged housings on the original tooling has not achieved wide acceptance.
Another approach which has been employed to some extent involves the attempted processing of the power steering housings on a conventional boring machine. In this instance the two ends of the housing, i.e., the counter bore openings, are aligned and then the housing is retained while an enlarged rebore is done at the end having the bore housing the spool valve. This approach presumes that the two counterbores are exactly concentrically aligned; however, it is to be noted that the two bores are not in fact concentrically aligned in the actual housings which are manufactured. Therefore, the error by way of initial misalignment of the bores is coupled with whatever error may exist by way of effecting the aforesaid end to end alignment. For these reasons substantial difficulties have been encountered in endeavoring to locate and effect rebore on conventional milling machines.
To applicant's knowledge no method and apparatus have been devised which will obviate the above discussed problems of achieving sufficient accuracy in the reboring of non-aligned counterbores in a workpiece to quickly and repeatably effect precision reboring for extremely close tolerance applications. The aforedescribed methods and apparatus not only fail to achieve sufficient precision in a large number of instances but also tend to be slow and time consuming, thus increasing the cost of endeavoring to repair workpieces of this type.